Waterborne contaminants e.g., toxic chemical substances and pathogens, are a primary public health concern in developing countries and result in millions of deaths every year [1]. Minimal drinking water treatment is beneficial and should include removal of harmful contaminants such as organic molecules, ions, and pathogens.
Electrochemical processes have been reported to inactivate both viruses and bacteria [5-7]. Most previous studies have focused on electrochemical generation of active chlorine species (>2.5 V; HOCl, Cl2.−) or electrochlorination [6, 8]. However, active chlorine-based pathogen inactivation can result in formation of harmful disinfection by-products [9], making the treated water unsuitable for drinking. As such, boron-doped diamond (BDD) anodes have been developed for bacterial inactivation. Although BDD anodes do not generate active chlorine species [10, 11], they require greater driving potentials (>3.0 V) than electrochlorination and thus increase energetic requirements for the disinfection process. Another alternative material for electrochemical disinfection is porous elemental carbon. Carbon cloth [12], carbon fiber [13], and granular activated carbon [14] anodes have been reported to be useful for electrochemical inactivation of attached bacteria at relatively low potentials (˜1 V). While the low driving potentials of these carbon-based anodes may reduce energy requirements and avoid disinfection by-product formation, these porous elemental carbon anodes do not have large specific surface area for efficient electrochemical processes. Previous research has also discussed destruction of organic compounds by electrochemical oxidation. However, the low mass transfer of contaminants from water to the electrode surface has limited the usefulness of electrochemical techniques in water treatment.
A recent study has been attempted to improve the overall mass transfer of chemical compounds in electrochemical treatment of contaminated water. Yang J. et al., 43 Environ. Sci. Technol. 3796 (2009). The Yang J. et al.'s system utilizes electrodes made of carbon nanotubes (CNTs) packed between two activated carbon fiber felts. Such system has been shown to degrade an organic dye (e.g., X-3B) present in water by re-circulating the contaminated water through the system for ˜90 mins at an applied potential of about 10V. However, re-circulation of contaminated water through the system limits its usefulness in continuous free-flow processes. Further, the Yang et al. reference does not disclose the ability of the system to remove biological microorganisms such as pathogens in in aqueous fluid.
Other studies have also previously reported that CNTs can be useful for adsorbing ionic dyes (7), chlorophenols (8), and natural organic matter via van der Waals interactions with the sp2-conjugated (planar) CNT sidewalls (9). CNT oxidation produces a large number of carboxylate surface groups that can bind metal ions such as Zn2+ and Cd2+ (10). CNTs coated with ceria have been utilized to separate chromium and arsenate from aqueous solutions (11, 12). Further, randomly-oriented single-walled carbon nanotube (SWNT) (14, 15) and multi-walled carbon nanotube (MWNT) (16) filters have been previously shown to isolate bacteria and virus from an aqueous fluid by sieving and depth filtration, respectively. Aligned MWNT network can also be useful for isolation of heavy petroleum hydrocarbons, bacteria, and virus from aqueous solution by gravity filtration through their interstitial space (19). Although the CNTs have been used to separate organic matters and bacteria from an aqueous fluid, e.g., by adsorption and filtration (mainly size exclusion), adsorption breakthrough can occur over time. Unless the adsorbed/sieved matters on the CNTs are destroyed and/or removed, the over-loaded CNTs would be rendered ineffective for further filtration. In addition, the adsorbed/sieved organics and pathogens may remain active, toxic, and/or viable. If they are not inactivated or degraded, the adsorbed matters can still pose potential health hazards in our environment.
The application of electrochemical processes in water treatment has drawn considerable attention in the past few years, because the electrolytic process is easy to control by potential and current, and such process can operate at low temperatures and pressures. However, the electrochemical technique is not widely applied in water treatment because of the high cost and low current efficiency caused by low contaminant mass transfer from water to the electrode surface. While CNT is an attractive material for aqueous filtration due to large specific surface area, adsorption breakthrough poses a limitation on the filter life-time and its usage in continuous water treatment processes. As such, there is a strong need to develop a more effective and efficient apparatuses and/or methods for water treatment. Further, there is an unmet need in the art for development of novel point-of-use water filtration devices and methods for removal and/or inactivation of waterborne pathogens and/or contaminants.